This application relates to power resistors. More particularly, the application relates to power resistors commonly referred to as chip power resistors. Specifically, it relates to limiting heat flow in chip power resistors having a continuous power density greater than about 20 watts per square inch so that while that portion of the resistor where the resistive element is located remains rather hot, the terminals of the resistor, the points of mechanical attachment of the terminals to the chips, and in particular the points of electrical contact between the terminals and the resistive zone remain relatively cool. Thus, whereas many electronic components seek to dissipate heat, we seek to prevent heat dissipation for reasons which will become readily apparent from the ensuing discussion.
For economy of both weight and space it is highly desirable to have electronic components as compact as possible. In the case of resistors, this means cramming a given resistance into an increasingly smaller space. Where the resistor is a chip resistor, which is a thin, flat, wafer-like article usually of rectangular shape, the thickness of the resistor is small relative to its other dimensions and the thickness of the resistive portion, generally deposited as a film on the chip substrate, is even more negligible relative to its other dimensions. Consequently, one is effectively packing a given resistance into smaller and smaller areas as the chip size decreases and increasing the power density as the unit becomes smaller. Attending this increase in power density is an increase in surface operating temperature in that region containing the resistive element, and until relatively recently the availability of economical chip substrates which could withstand the thermal extremes of cyclic on-off operation provided a practical limitation to decreasing resistor size.
In recent years materials with suitable coefficients of thermal expansion and with excellent mechanical and electrical properties have become commonly available, and when used as chip substrates these have afforded the opportunity to further decrease resistor size. Exemplars of such materials are porcelain or glass-coated metals. For example, the substrates of Hang et al., U.S. Pat. No. 4,256,796 are comprised of a metal core, such as steel, coated with a porcelain. The porcelain components are applied to the metal core and fired to provide a partially devitrified porcelain coating on the metal where the resulting substrate has a deformation temperature of at least 700.degree. C. and a coefficient of thermal expansion of at least 110.times.10.sup.-7 /.degree. C. Such substrates when used as base materials for chip resistors permit fabrication of resistors having a continuous power density greater than 20 watts per square inch. But accompanying this increase in power density are other features whose origins and effects require a brief excursion into the land of chip resistor fabrication to better understand the problem with which we are faced.
FIG. 1 is a general schematic representation of a chip resistor. The underlying chip substrate, 1, has a resistive region, 2, which generally is a film of a conductor or semiconductor deposited on the chip surface. The resistive region is bounded by conductive strips, 3, which are in electrical contact with the resistive region. To the chip substrate are securely attached terminals, 4, which are in electrical contact with the conducting strips, 3, most often via a solder junction here represented as 5. All or part of this assembly may have an overglaze or glass coating which affords mechanical and environmental protection to the assembly elements. However, as this protective feature is unrelated to our invention, we shall not refer to it any further.
At the high power densities of interest here the shaded resistive region may easily attain temperatures of about 350.degree. C. This presents no problem for the underlying chip substrate, since the material was developed to readily withstand such temperatures and the thermal shock attending frequent and rapid cycling between ambient temperature and 350.degree. C. However, especially where the substrate is a porcelain coated metal, there is heat transfer from the resistive region of the chip to the terminals region, largely via conduction through the metal. Thus, the terminals may readily attain temperatures greatly in excess of 100.degree. C., which constitutes a major problem since many circuit boards into which the chip resistors may be incorporated deteriorate at temperatures over about 100.degree. C. Furthermore, solder connections also begin to deteriorate at a temperature in the region of 150.degree.-200.degree. C., which is a temperature readily attained in the high power density chip resistors under discussion, leading to a variable and uncertain resistance value and finally an open circuit at the junction of the terminals and the conductive strips.
The aforementioned problems are so severe that in our experience chip resistors with a power density of 20 watts per square inch or greater usually fail after only several days use, which is unquestionably an unacceptable performance standard. The result to be achieved was clear; decrease heat transfer from the 350.degree. C. resistive region of the chip resistor so that the terminals remain under about 100.degree. C. and the solder junction of the terminal to the resistance portion remain under about 175.degree. C., and attain this result without any significant change in chip size, chip weight, or chip electrical performance. This application is directed toward a relatively simple solution to the problem.
As previously stated, heat transfer from the resistive region of the chip to the terminal region occurs largely via conduction through the chip substrate, and where a porcelain coated metal is the chip substrate the heat conducting medium is mainly the metal, since the metal is a far better heat conductor than is the porcelain overcoat. The solution to the problem is to reduce heat conduction from the heat generating resistive region of the chip resistor to the terminal region(s) of the resistor. Stated differently, a general solution utilizes locating means for reducing heat conduction between the resistive (heat source) and terminal (heat target) regions of the chip resistor. We have further found that an air gap is an effective, convenient, and inexpensive means for reducing heat conduction, i.e., that one can effectively limit heat transfer between the resistive and terminal regions of a chip resistor by having an air gap between the regions. In effect, the air gap acts as an insulator limiting heat flow by restricting the cross-sectional area of the "heat pipe" between the two regions. Stated differently, the air gap of our invention increases thermal resistance in the zone between the resistive and terminal regions, thereby decreasing heat flow from its source to the terminals.
The prior art does not appear to have any teachings suggesting our general or our specific solution to the stated problem, nor does there seem to be teachings in any way related thereto. For example, U.S. Pat. No. 3,497,859 teaches a planar resistor having a recess at its underside to provide a gap between the mounted resistor and the surface of a printed circuit board. The recess was provided to help distribute current over the face of the body and to provide a passage between the resistor and the circuit board for one or more electrical conductors to pass without contacting the resistor. The recess also made it unnecessary to cover the resistor with electrical insulation. It can be readily seen that there are critical and important functional differences between the patentees' teachings and our invention to be described. First, the gap is between a mounted resistor and a printed circuit board, and is not on the chip resistor itself. Secondly, the gap serves entirely different purposes wholly unrelated to that in our invention. It is clear that the skilled worker having this reference before him would have no inkling of the solution to the problem applicants faced.
Worth et al. in U.S. Pat. No. 4,333,069 show a wire wound resistor whose terminals have a gap therein for decreasing the weight and increasing the flexibility of the resistor in order to allow spatial adjustments for openings in printed circuit boards. Takayanagi, U.S. Pat. No. 4,658,234, discloses a resistor network having a plurality of resistor elements disposed parallel to each other in an insulation substrate and enclosed in a resin seal that encloses both the substrate and the resistor elements. The resin enclosure prevents effective heat dissipation leading to deteriorating performance. This was solved in part by providing the resin seal with a plurality of heat-dissipating holes extending through the resin seal and formed between the selected resistor elements. It is readily seen that the patentees "holes" offer the purpose of heat dissipation, which is precisely opposite the function of the air gap in our invention.